The goal of this project is to quantify the acute and chronic effects of exercise on hemodynamic conditions in the infrarenal aorta of human subjects with small AAA (AAA diameter equal to or >3, equal to or <5 cm). The subjects will be a subset of the patients in the standard therapy arm and exercise intervention arm described in Specific Aim 2 of Project IV: Evaluation of Exercise Therapy for Small AAA. We will test the hypotheses that: (i) Shape matters: Differences in shear and dynamic tensile forces acting on the vessel wall, resulting from differences in aneurysm shape, are predictive of AAA growth rate, (ii) Size matters: As AAA enlarge, adverse hemodynamic conditions (including regions of low mean wall shear stress and high particle residence time) are exacerbated under resting conditions, (iii) Structure and motion matter: Differences in wall thickness, tissue composition, cyclic wall motion, and fluid-solid interactions affect AAA enlargement, (iv) Exercise matters: Increased infrarenal blood flow resulting from acute lower limb exercise, eliminates regions of adverse hemodynamic conditions, dramatically increasing wall shear stress and reducing particle residence time in all subjects regardless of AAA shape or size, (v) Persistence matters: Regular exercise slows AAA progression affecting size and shape, and results in more favorable hemodynamics and vessel wall motion. We will test these hypotheses by quantifying hemodynamics and wall tensile stresses under resting and exercise conditions in the abdominal aorta of patients with small AAA randomized to chronic exercise therapy or standard therapy. Our specific aims are: (1) Quantify time-varying abdominal aortic anatomy in AAA patients, (2) Quantify abdominal aortic blood flow at rest and during dynamic exercise using a custom MR-compatible bike in a 0.5T open MRI, and (3) Develop and validate computational methods to model blood flow, pressure, and wall motion in "patient-specific" computational models of the abdominal aorta of patients with small AAA randomized to chronic exercise therapy or standard therapy.